1. Field of the Invention
The inventions relate generally to optical telecommunications and, more particularly, to electro-optical modulators.
2. Discussion of the Related Art
One goal of telecommunications research is to increase transmission rates for digital data. Presently, optical transmitters transmit digital data at rates that are below desired values. The transmission rate of an optical transmitter is, in part, limited by the frequency at which a modulator is able to encode data onto a continuous optical carrier wave.
Typically, modulators amplitude-encode data onto the optical carrier wave using microwave or millimeter control waves. The control waves electro-optically modulate refractive indexes in optical waveguides of the modulator. The optical waveguides form arms of an optical interferometer. By modulating refractive indexes, the control waves modulate propagation times in the arms of the interferometer and thus, intensity distributions produced by the interferometer. From the interferometer, the pattern is sent to an output optical waveguide whose coupling depends on the received intensity distribution. Thus, the optical wave transmitted by the output waveguide, i.e., the modulator""s output optical wave is amplitude-modulated by the control waves that control the form of the interference pattern.
For optimal modulation, velocities of the control and carrier waves should be matched in the electro-optical modulator. Otherwise, wavefronts of the control wave corresponding to different data periods will modulate the same portion of the optical carrier wave. The accuracy of the match between velocities of control and optical carrier waves provides an upper limit on the operating frequency of an electro-optical modulator.
In optical modulators, mismatches between control and optical carrier wave velocities occur for several reasons. First, optical wavelengths are typically between about 1.3 microns and about 1.7 microns and control wave wavelengths are typically in the centimeter to submillimeter. Since refractive indexes of the modulator""s dielectrics depend on wavelength, this wavelength dependence tends to produce a velocity mismatch between the optical carrier and control waves. Second, control waves include fringe field components, i.e., fields in air or vacuum. Since propagation velocities are higher in air and vacuum, fringe field components tend to make control-wave velocities higher than those of optical carrier waves, which propagate in condensed dielectrics of the electro-optical modulator.
In one aspect, methods are provided for operating electro-optical modulators that reduce desynchronization between control and optical carrier waves therein.
One embodiment features a method that includes transmitting a sequence of wavefronts of an optical carrier wave having a first wavelength to an optical waveguide and transmitting a control wave having a second wavelength to a control waveguide. The control wave electro-optically modulates velocities of the wavefronts in the optical waveguide. A dielectric cladding adjacent the optical waveguide has a refractive index at the second wavelength that is larger than the refractive index in the optical waveguide at the first wavelength.